This invention relates to a large-current fuse unit of a cartridge type used in an electric circuit in an automobile or the like, and more particularly to such a fuse unit having a temperature fuse provided in the vicinity of a fusible portion of a main fuse.
A fuse 1 of a cartridge type as shown in FIG. 5 has heretofore been used in an electric circuit of an automobile or the like. This fuse comprises a pair of terminals 3 and 3 interconnected by a fuse element 5, a housing 7 made of an insulative thermal-resistant resin and holding the terminals 3 and the fuse element 5 therein, and a transparent cover 11 closing an open top 9 of the housing 7. Terminal receiving chambers for respectively receiving the terminals 3 and 3, as well as an element receiving space communicating with these terminal receiving chambers, are formed within the housing 7. When the terminals 3 and 3 are received respectively in the terminal receiving chambers, the fuse element 5 is positioned in the element receiving space, so that whether or not the fuse element is melted can be confirmed with eyes through the transparent cover 11. When a current larger than a rating flows through the fuse element 5, the fuse element 5 is melted by heat, generated therein, to open the circuit, thereby protecting a wire and an equipment.
Generally, in the above conventional fuse, there is the correlation between an energizing current and a melting time as shown in FIG. 6. More specifically, the fusible portion is instantaneously melted by a current larger than 200% of the rating of the fuse, but the melting time is relatively long with a current less than 200% of the fuse rating since the fuse is designed to withstand a rush current. When such current as is produced upon discontinuous short-circuiting (rare short circuit) flows instead of the continuous flowing of the current, the fusible portion of the fuse element 5 repeatedly generates and dissipate heat, so that the melting time tends to become long. On the other hand, when the discontinuous short-circuiting current flows through the wire constituting the circuit, the wire fails to dissipate heat as in the fusible portion even when the current is interrupted since the wire is covered with a sheath, and therefore the temperature of the wire continues to rise because of the accumulated heat, and in the worst case, there is a possibility that the wire produces smoke.